Geochemical mapping of organic carbon in stalagmites using liquid-phase and solid-phase fluorescence

Résumé En

The soil-derived organic matter incorporated in speleothems provides a proxy for the impacts of climate and environmental changes on the surrounding catchment. These organic proxies, combined with inorganic proxies, can be used to investigate variations in carbon fluxes. The present article describes a method for obtaining high spatial-resolution estimations of speleothem organic carbon concentrations by using the relationship between liquid phase fluorescence and the organic carbon contents of soil samples. Applying this method to soil and stalagmite samples from three locations in the French Prealps gave estimated stalagmite carbon concentrations of between 0.27 and 3.03 mg C/g of calcite, which are consistent with measured concentrations reported in the literature. We assessed the high-resolution reliability of our procedure by comparing TOC estimations with solid phase fluorescence values for samples taken every 2-mm along one of the stalagmites (TAM). Due to variations in the optical properties (e.g., optical density) of the calcite, revealed by Near Infra Red Reflectance, it was necessary to draw up a nonlinear model in order to obtain good estimates (R2 = 0.81) of organic carbon concentrations from solid-phase fluorescence results. The resulting high-resolution map of organic carbon concentrations along the TAM sample was consistent with the recent history of the area's environment. Our results show that variations in carbon flux in mountain karst environments are strongly linked to changes affecting the area's soils.